Vehicle detection

ABSTRACT

A vehicle detection and classification system which comprises a plurality of proximity sensors is distributed in a fixed spatial array relative to a road such that a distance of each sensor to the nearest adjacent sensor is less than a minimum horizontal dimension of a vehicle to be detected. The array has a maximum dimension greater than the minimum horizontal dimension of a vehicle to be detected. Each of the sensors is configured to determine presence or absence of a vehicle and to communicate data regarding said presence determination to a data processing system, wherein the data processing system is configured to use data from a plurality of the sensors to detect and classify a vehicle on the road based on at least one dimension of the vehicle.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from pending United KingdomPatent Application No. 1503855.7 filed Mar. 6, 2015, which isincorporated herein by reference.

BACKGROUND

This application relates to the detection and classification of vehiclestravelling on a road.

In modern road networks it is often necessary to determine the types ofvehicles on a given road for a number of reasons. It is particularlyuseful at toll points to be able to determine automatically the type ofvehicle approaching the barrier so that an appropriate amount can becharged to the driver according to the type of vehicle in question. Afurther application for the detection and classification of vehicles isin traffic monitoring systems, where it is useful to the operator of theroad network to be able to determine the levels of traffic and thevehicle composition of the traffic in order to make strategic decisionsrelating to the operation of the roads.

Conventional systems used for this purpose often rely on eitherinductive sensors that count the number of wheel axles present on avehicle, or utilise cameras alongside image processing techniques toclassify vehicles. However, these systems give rise to a number ofproblems which the present invention seeks to address.

A typical inductive system comprises an inductive loop that operatesusing induction to detect the wheel axles as they pass the loop. Suchsystems however are prone to issues when multiple vehicles pass thesensor in quick succession, as they cannot distinguish betweenindividual vehicles in bumper-to-bumper traffic, instead often detectingvery long singular vehicles. Furthermore, only vehicles larger than aparticular size can be detected, making it difficult to detect andclassify bicycles, scooters and motorcycles, and false positivedetections are not uncommon.

Systems that utilise optical techniques such as laser sensors or camerashave difficulty when visibility is poor, which is often the case innon-ideal weather conditions such as rain, snow and fog. However, hotweather can also be a problem as fumes from the road surface (normallymade of asphalt) can dramatically hinder the performance of such opticalsystems. Cameras can often suffer problems with occlusion whereby,depending on the placement of the camera and the relative positions ofthe vehicles, a first vehicle may obstruct the view of a second vehicle,preventing the proper detection and classification of the vehicles.Laser based systems often struggle to differentiate between a fast longvehicle and a slower shorter vehicle. Furthermore, even with decreasingcosts relating to optical devices in recent years, the physicalimplementations of these systems can be complex and expensive.

Both inductive loop and laser based systems are also usually calibratedor optimised for a particular range of speeds and require the vehiclesto remain in a particular lane whilst being detected. However, theApplicant has appreciated that it would be advantageous to be able todetect and classify vehicles driving at any speed, in any drivingpattern, in any prevailing weather conditions.

SUMMARY

The present invention seeks to provide an alternative system that canfor example be used in place of or to enhance conventional systems knownin the art.

When viewed from a first aspect, the present invention provides avehicle detection and classification system comprising a plurality ofproximity sensors distributed in a fixed spatial array relative to aroad such that a distance of each sensor to the nearest adjacent sensoris less than a minimum horizontal dimension of a vehicle to be detectedbut the array has a maximum dimension greater than said minimumhorizontal dimension of a vehicle to be detected, each of said sensorsbeing configured to determine presence or absence of a vehicle and tocommunicate data regarding said presence determination to a dataprocessing system, said data processing system being configured to usedata from a plurality of said sensors to detect and classify a vehicleon said road based on at least one dimension of said vehicle.

It will be appreciated by a person skilled in the art that in accordancewith the invention, the vehicle detection and classification systemprovides an advantageous arrangement for determining one or moreparameters associated with individual vehicles in proximity of thesensor array. The resolution of the sensor array is such that thesensors are close enough together to be able to resolve particularvehicles of interest while the extent of the sensor array is wideenough, at least in preferred embodiments, to provide adequate detectionof vehicles of interest in any lane on a multi-lane road, as well asvehicles that are changing between lanes whilst traversing the sensorarray. Embodiments of the invention could be used just forclassification of vehicles into different types. However embodiments ofthe invention are additionally or alternatively able to determine aspeed and/or angle of travel of a vehicle.

Although the separation between the sensors can be varied to suit aparticular application, the distances are typically in the range ofbetween 0.1 m and 2 m, e.g., between 0.2 m and 1 m. The separation maybe different in one direction compared to another (e.g., with sensorsspaced closer together in a longitudinal direction than a transversedirection). The spacings in one or both directions could be regular,varying or irregular. The array could have a square, rectangle, diamond,interleaved or any other pattern.

There are a number of possible configurations for such an array ofsensors. In a set of embodiments the array of sensors is provided in oneor more planes parallel to the surface of the road. This arrangement isparticularly advantageous for ease of detection.

In a set of embodiments the array of sensors is disposed beneath orflush with the surface of the road. The Applicant has appreciated thatit would be particularly advantageous to place the sensor array beneathor flush with the road, such that vehicles traverse over the sensors,minimising the amount of additional infrastructure needed to implementthe system.

The Applicant has also appreciated that there are situations where itwould be advantageous to arrange the sensor array such that it isdisposed above the road surface, such as in overhead gantries or on theceiling of a tunnel. In a set of embodiments therefore the array ofsensors is disposed above the surface of the road such that vehiclespass beneath them. Of course in a practical system a mixture of sucharrangements might be employed.

There are a number of configurations possible for the relative spatialresolutions of the sensor array and the individual sensors, i.e., thevertical and horizontal spacings between individual sensors within thearray compared to the coverage of individual sensors. The Applicant hasappreciated that it is not necessary for example to provide blanketcoverage of a piece of road as long as vehicles can be unambiguouslyclassified.

Each of the sensors within the sensor array communicates in some waywith the data processing system, such that determinations can be made asto the presence of a vehicle in proximity to said sensor. In a set ofembodiments at least some of the sensors are configured to communicatesaid data to said data processing system via at least some othersensors. Such a relay arrangement may be beneficial in reducing thenumber of interconnections required, or the wireless transmission rangeof individual sensors, which may have an advantageous impact on cost andbattery life.

The data processing system could comprise a centralised unit. In someembodiments however, said data processing system is at least partiallydistributed across the array of sensors. The Applicant has appreciatedthat it may be advantageous to provide the sensor array with the abilityto perform data processing locally by distributing computations acrossthe sensor array. Intermediate arrangements are also possible whereby amaster sensor carries out some processing for a localised group. Thesensors may communicate with each other to share data in order tocarrying out data processing.

There are a wide range of technologies that would be possible to use inthe implementation of the present invention, such as ultrasound,infrared and radar based sensors. However, in a set of embodiments thesensors comprise ultra-wideband radar sensors. Ultra-wideband radar isparticularly advantageous for its ability to penetrate materials such assnow, ice and gravel where radar sensors with a smaller bandwidth wouldfail. Additional benefits of such ultra-wideband radar sensors are thatthey provide excellent time-resolution and can detect objects atrelatively short distances.

The sensors could be provided with power from an external source, but ina set of embodiments, the sensors are battery powered. This reduces thecost of implementation, especially in a set of embodiments wherein thesensors are operated in a burst mode. In a set of embodiments, eachburst comprises a pulse train modulated with a Direct Sequence SpreadSpectrum (DSSS) code, a known signal having the characteristics ofpseudo-random noise. In one example the chip rate may be between 1 and100 MHz. In such embodiments, the signals transmitted by the sensorcomprise discontinuous bursts of discontinuous pulses. Since the sensoris then transmitting for only a fraction of the time (i.e., it has arelatively low duty cycle), a significant reduction in power consumptioncan be achieved. A person skilled in the art will appreciate thisexample is non-limiting and other such configurations are possible.

In a set of embodiments ultra-wideband sensors are arranged to operatein the frequency range between 3.1 and 10.6 GHz. In a set of embodimentsthe sensors are in compliance with the frequency range (3.4 to 4.9 GHz)authorised for unlicensed use by the European TelecommunicationsStandards Institute (ETSI) in Europe. In another, potentiallyoverlapping, set of embodiments, the sensors are in compliance with thefrequency range authorised for unlicensed outdoor use by the FederalCommunications Commission (FCC) in the U.S.

More generally, as will be appreciated by those skilled in the art, theexact frequency ranges authorised for particular uses vary from countryto country and even over time. For example the sensors may operate inbands at 6 GHz, 24 GHz or 77 GHz to give just some further examples.References to ultra-wideband radar herein should not therefore beconstrued as being limited to a particular frequency range or regulatorylimits.

A further advantage of utilising ultra-wideband radar technology thathas been appreciated by the Applicant is that the sensors can, ifdesired, be arranged to communicate with one another using modulatedultra-wideband radar signals. For example, this advantageously providesthe ability for the individual sensors to synchronise their internalclocks. In some further sets of embodiments therefore the ultra-widebandsensors can communicate with each other using ultra-wideband signals.

There are a number of dimensions to a given vehicle that may beindicative of its type to aid in classification. The length and width ofa vehicle are often indicative of the type of vehicle in question. Bytaking the data collected by the sensor array, the data processingdevice can determine the length and/or the width of a vehicle as itpasses the sensor array. In some sets of embodiments said dataprocessing system is arranged to determine a length of the vehicle. Insome sets of embodiments said data processing system is arranged todetermine a width of the vehicle.

As mentioned above in a set of embodiments the data processing system isarranged to determine a speed of the vehicle—that is the absolutemagnitude of its rate of movement. This may be useful particularly intraffic monitoring applications, though a person skilled in the art willappreciate that this would also be advantageous in other applications.

Similarly in a set of embodiments the data processing system is arrangedto determine an angle or direction of travel of the vehicle. This mayalso be useful in traffic monitoring applications as explained below,though a person skilled in the art will again appreciate that this wouldalso be advantageous in other applications.

In a set of preferred embodiments the invention is implemented on aso-called multi-lane free flow toll area. The system provided inaccordance with the invention may advantageously allow vehicles to betracked even when travelling across lanes while approaching the tollarea. Such a tolling system might comprise a camera and an RF tagreader, and data from the sensor array can advantageously resolvesituations where the camera has detected a vehicle in a first lane whilethe RF tag reader has detected the same vehicle in a second lane at asubsequent time after said vehicle has moved from the first lane to thesecond lane, preventing the vehicle being charged twice by the tollingsystem. In contrast in some current traffic monitoring applications suchas tolling stations, vehicles changing lanes can cause issues asconventional systems cannot easily track the direction and angle oftravel associated with a given vehicle. This can result for example in avehicle being charged twice if it is detected in two lanes. The problemcan be avoided by providing separate individual lanes but these can havea negative impact on traffic flow.

In a set of embodiments the sensors can measure a time between atransmitted signal and a reflection of that signal being received togive a time of flight and therefore distance to the reflecting object.This can be used to determine a distance to the vehicle or other objectthat caused the reflection.

In a set of embodiments the sensors are arranged to detect wheel axleusage. This advantageous arrangement provides a way to distinguishbetween raised and lowered axles from the relative height differencebetween lowered and raised wheels and charge the appropriate feeaccordingly. In some tolling systems, the fee charged to a vehiclepassing the tolling station is calculated based upon how many axles arein use. The axles of some vehicles can be raised and lowered as requireddepending on the current load being transported by the vehicle. Forexample, in some tolling systems, a truck utilising three axles might becharged a lower fee than a truck utilising four axles, despite thetrucks having the same dimensions.

It may be possible to determine the classification of a vehicle strictlyfrom the geometry of said vehicle. However in a set of embodiments thedata processing system is arranged to determine a number of axlespresent on the vehicle. This advantageous arrangement may give furtherconfidence in classification decisions made, as typically trucks willhave a larger number of wheel axles than a car or motorcycle. This alsoadvantageously allows a truck classification to be further refineddepending on the number of axles it has, which may be useful in tollingstations where different toll charges are levied depending on the numberof axles a truck has.

It will be appreciated by a person skilled in the art that the analysisrequired to perform detection and classification of individual vehiclesmay be performed externally at a remote location utilising the data fromthe sensor array. Thus when viewed from a second aspect, the presentinvention provides a vehicle sensing system comprising a plurality ofproximity sensors distributed in a fixed spatial array relative to aroad such that a distance of each sensor to the nearest adjacent sensoris less than a minimum horizontal dimension of a vehicle to be detectedbut the maximum dimension of the array is greater than said minimumhorizontal dimension of a vehicle to be detected, each of said sensorsbeing configured to determine presence or absence of a vehicle and inuse to communicate data regarding said presence determination to a dataprocessing system, said data being such as to allow said data processingsystem to use data from a plurality of said sensors to detect andclassify a vehicle on said road based on at least one dimension of saidvehicle.

BRIEF DESCRIPTION OF DRAWING

An embodiment of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is an overhead view of an embodiment of the invention;

FIG. 2 shows an example of a sensor in accordance with an embodiment ofthe invention;

FIG. 3 is a block diagram of a sensor in accordance with an embodimentof the invention;

FIG. 4 shows a typical burst mode of operation of a sensor in accordancewith an embodiment of the invention;

FIG. 5 shows a typical transmission and reception of a burst utilised bya sensor in accordance with an embodiment of the invention;

FIG. 6 is a further view of an embodiment of the invention wherein avehicle has entered the detection zone of the sensor array;

FIG. 7 is a side-on view of an embodiment of the invention;

FIG. 8 is a graph illustrating the operation of an embodiment of theinvention;

FIG. 9 is an overhead view of an embodiment of the invention where afirst vehicle is a truck and a second vehicle is travelling at anoblique angle to the road direction; and

FIG. 10 shows the relative spacings between the sensors compared tovehicle sizes in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows an overhead view of an embodiment of the invention. A road102 comprises two lanes 108, 110 with the same direction of trafficflow. A sensor array 112 is arranged such that it straddles both lanes108, 110.

The sensor array 112 comprises a number of individual high resolutionradar (e.g., ultra-wideband, or UWB) sensors 114. A data processingsystem 116 is connected to, and takes data from the sensor array 112.Travelling in the two lanes 108, 110 are two vehicles 104, 106. At thismoment in time, neither of the vehicles 104, 106 is within range of thesensor array 112 and thus no vehicles are detected at this time.

The data processing system 116 in this example is a dedicated roadsideunit that takes data from the sensor array 112 and performs thenecessary processing for vehicle detection, tracking and classificationas will be described below with regard to the other Figures. However inother embodiments the data processing system could comprise a computeror computer network at a remote location. The data processing systemcould also be distributed across the sensor array 112, with each sensor114 performing local processing using an onboard processor. Typicallythis would be on the basis of data generated by the sensor itself anddata received from other sensors.

The individual sensors 114 within the sensor array 112 need not connectseparately and individually to the data processing system 116 butinstead are networked such that each sensor 114 has either a directconnection to the data processing system 116, or an indirect connectionvia at least one other sensor. Any network topology known in the art perse may be used including ring networks, star networks, bus networks,multi-hop routing etc.

FIG. 2 shows an example of the sensor in accordance with an embodimentof the invention. The sensor 114 shown is particularly suitable formounting within a road surface but a person skilled in the art willappreciate that this same sensor could be utilised in other arrangementssuch as mounting in the ceiling of a tunnel, on a gantry, platform etc.or freely suspended above a road, track, lane etc. The sensor 114 has ashallow cylindrical shape with a depth 216 and diameter 222 that aresuitable for the chosen type of mounting. The particular physical formof the sensor is purely exemplary and other shapes or sizes couldequally be employed. By providing a recess in the road surface ofsubstantially the same depth 216 and diameter 222, this sensor 114 canbe mounted flush to the road surface such that vehicles can drive overthe sensor array with minimal impact. The body of the sensor 114 issealed to prevent ingress of dirt, moisture etc.

The shape of the sensor 14 allows radar signals to pass through the topsurface 218 in an outward radial direction. The sensor 14 contains anantenna (not shown) for transmitting and receiving the signals arrangedto radiate energy outwards through the top surface 218. The radiationdiverges from the top surface. For example the transmission may have alobe angle (i.e., the angle of divergence) of approximately forty fivedegrees.

Signals which are reflected—e.g., from a vehicle—also pass back throughthe top surface 218 to a suitable receiver inside the body of thesensor. The sensor 114 determines whether there is an object within itsdetection field depending on a number of factors that may include thetime between transmission and reception of signals as described below,the signal strength of the received signal, the frequency composition ofthe received signals etc.

The sensor 114 utilises a direct-sequence spread spectrum (DSSS)technique (known in the art per se) to transmit a very short RF widebandburst—e.g., having a burst width of the order of 10⁻³ seconds (onemillisecond), generated by multiplying a radio frequency carrier (i.e.,a pulse train) and a specific pseudo-random noise digital signal (oftenreferred to as “chips” in the art). The sensor 114 receives a reflectionof the burst and correlates it with the pseudo-random noise signal. Theoffset between the corresponding pseudo-random noise signal that wasencoded within the received reflection and the known pseudo-random noisesignal that was transmitted provides the time-delay associated with thepropagation of the signal. This time-delay corresponds to the distancebetween the sensor 114 and the reflector. For example a time delay of2×10⁻⁹ seconds (2 nanoseconds) would indicate a reflector 0.3 metersaway (and therefore a round trip distance of 0.6 meters) given that theradar signal travels at the speed of light (3×10⁸ m/s). Typically if avehicle is present, the distance between the sensor and the vehicle willbe between 0.1 and 0.5 m. Bursts can either be sent at regularintervals, e.g., at a rate of 100 bursts per second, or can be sent atrandom intervals to improve resistance to mutual interference betweensensors as well as to other forms of interference in general.

The sensor 114 in this embodiment is also provided with a cable 220 thatprovides it with the ability to communicate with other sensors withinthe array and/or a data processing system located at the roadside or atsome other remote location. This cable 220 may also provide power,though equally the sensor may be battery operated, utilising a batterydisposed within the sensor 114 itself.

FIG. 3 shows a block diagram of the sensor 214. This particular sensor114 comprises four distinct elements: a microcontroller 330,transmitter/receiver module 332, a battery 334, and a data communicationmodule 336.

The microcontroller 330 acts as the primary processing unit for thesensor 114, controlling operation of the transceiver module 332 and thedata communication module 336. The transceiver module 332 communicatesdata to the microcontroller 330 to allow presence of an object within apredetermined range to be determined. The nature of this data will bedescribed later below with reference to FIG. 6.

Depending on the configuration of the sensor array, the communicationmodule 336 may communicate directly with a data processing system at theroadside or at some remote location, or alternatively the sensor 114 maybe arranged to communicate with its peers within the array. The datacommunication module 336 interfaces with the cable 200 (not shown) butequally could enable wireless communication e.g., using IEEE 802.11,Bluetooth (Trade Mark), ZigBee, UWB, or any other such communicationtechnology. In this particular embodiment, the sensor 114 utilises highresolution radar to communicate with peer sensors, allowing the samehardware to be utilised for both vehicle detection and communication.

The battery 336 provided within the sensor 114 provides power to theentire unit and has a long lifetime such that it rarely requiresreplacement. It is also envisaged that it could be recharged e.g.,through a photo-voltaic module.

FIG. 4 shows a typical burst mode of operation of a sensor in accordancewith an embodiment of the invention. This ultra-wideband radar sensorutilises DSSS as discussed above with reference to FIG. 1. This Figureshows a plot of signal amplitude 400 as a function of time 402. Thesensor is operated using a DSSS ultra-wideband radar signal 404 that isactivated and deactivated for short bursts of time. This is shown as asingle frequency for the sake of clarity in FIG. 4.

As mentioned previously the bursts 406A, 406B, 406C have a very shortduration e.g., of the order of 1 millisecond, although this duration isnot shown to scale, again for clarity. The burst cycle length 408 mightbe 10 milliseconds to give a burst rate of 100 per second. Higher burstrates may give better resolution, enabling not only the detection of anobject's presence but also its speed as will be discussed below withreference to FIG. 8.

Each 1 ms burst 406A, 406B, 406C is shown in greater detail in the lowerhalf of FIG. 4. Each burst is constructed by multiplying 412 aduty-cycled pulse train 410 comprising very short pulses 414 of 1 nsduration and a 100 ns cycle 413, with a pseudo-random digital chipsequence 411 that takes digital values “0” or “1”. It will beappreciated that the rates supplied above are merely indicative and bothhigher and lower burst rates and pulse timings may still give usableestimates of speed and position of objects.

FIG. 5 shows a typical transmission and reception of a burst utilised bya sensor in accordance with an embodiment of the invention, showing thesignal power 510 as a function of time 512 for both transmitted andreceived signals. At an initial time 500 a burst 504 (corresponding tothe bursts 406A, 406B, 406C in FIG. 4) is transmitted by the sensorwhich propagates through the sensor housing and into the surroundingair. After a delay 508 at a subsequent time 502, a reflected burst 506is received by the sensor. The reflected peak is detected by correlatinga number of time-offset copies of the transmitted pseudo-random noisechip sequence with the received reflected signal and choosing thehighest correlation peak. The duration of the delay 508 corresponds tothe distance between the sensor and the object that caused the reflectedburst 506. Since the signal speed is known, the distance over which thesignal has travelled can be calculated. It will be appreciated that theburst shown in FIG. 5 is idealised, while in reality there will likelybe a number of received false echoes and the reflected burst 506 willlikely be spread out over a greater time period when compared to thetransmitted burst 504.

Operation of the system will now be described. FIG. 6 shows a viewsimilar to FIG. 1 in which one of the vehicles 106 has entered thedetection zone of the sensor array 112. The vehicle 106 is now proximateto (i.e., within the field of view of) a selection of the individualsensors 114 within the sensor array 112. Each of these individualsensors 114 reports to the data processing system 116 that there is anobject within its individual detection zone.

The measurements from all sensors 114 in the sensor array 112 areaccurately timestamped before being transmitted to the data processingsystem 116. The data processing system 116 then normalises all of themeasurements to a common time base and then analyses the data todetermine a moving bounding box associated with the vehicle 106. Fromthis bounding box the data processing system 116 obtains the speed anddirection of the vehicle. The length and width of the bounding boxalongside other parameters including the number of axles can then beused by the data processing system 116 to determine the classificationof the vehicle 106.

FIG. 7 shows a side-on view of the situation described above withreference to FIG. 6 wherein a vehicle 106 is driving over the sensorarray 112. While the sensor array 112 extends beyond the length of thevehicle 106, only the sensors between the rearmost covered sensor 120and the foremost covered sensor 122 detect the vehicle 106 at thismoment in time. Any further sensors within the sensor array 112 that arenot substantially covered by the vehicle 106 do not detect the presenceof this particular vehicle at this time, but there may be other separatevehicles on the road 102 being detected by other portions of the sensorarray 122 simultaneously.

FIG. 8 shows a graph illustrating the operation of an embodiment of theinvention. On the y-axis is detected height 800, and on the x-axis istime 802. Shown on the graph is a trace 812 of the detected height overtime for an individual sensor within a sensor array in accordance withthe present invention.

When the sensor detects that the height 800 is beyond a particularthreshold value, it is determined that no vehicle is proximate to thesensor. This detected ‘height’ (i.e., an echo distance determined by theradar) may not be to a particular object but is indicative of the extentthat received reflections of transmitted signals have travelled and thatas it is sufficiently far, no vehicle is within the detection zone ofthe sensor.

When no object is proximate to the sensor, the detected height floatsaround an average “background noise” level that is largely determined bythe environment surrounding the sensor. It is noteworthy that regardlessof the presence of an object within the sensor's field of view, thenature of the changing environment will lead to fluctuations in thedetection signal as is apparent on the trace 812.

At an initial time 804, a vehicle is not yet over but is approaching thesensor. This causes the detected height 800 to start reducing over timeas the vehicle gets closer.

At a later time 806, the sensor is completely obstructed by the vehicleand the detected height 800 drops to a value below a given thresholdsuch that the sensor determines that a vehicle is present within itsdetection zone. The sensor subsequently reports to a data processingsystem or its peers that an object is present.

After a while, at time 808, the vehicle begins to exit the detectionzone of the sensor and the detected height 800 begins to increase again,tending toward the background noise level, which is reached around time810 when it is determined that the vehicle is no longer proximate to thesensor.

Each of the sensors within the sensor array will have its owntime-varying detected height trace, and by combining the results fromthe entire array of sensors, it is then possible to determine numerousproperties of the vehicle(s) within the area covered by the sensorarray. For example, from the groupings of sensors in proximity to oneanother that all report that they have a vehicle within their detectionzone, a bounding box can be defined that describes the geometry of avehicle that is within the confines of the sensor array. Once thisbounding box is defined, the length and width of the box indicate thelength and width of the vehicle that gave rise to said bounding box.

Furthermore, from the gradient of the trace 812 between transitions suchas between points 804 and 806 or between 808 and 810 it is possible todetermine the speed of the vehicle. However, in order to do so,typically edge information from multiple adjacent sensors will need tobe combined. It is also possible to estimate the speed of the vehicleusing Doppler shifts in the frequency of the received signals. Using theDoppler shifts in frequency to estimate the speed of the vehicle alsomeans a lower burst rate can be used.

FIG. 9 shows an overhead view of an embodiment of the invention where afirst vehicle 904 is a truck and a second vehicle 906 is travelling atan oblique angle to the road direction.

In one lane 908 of the road 902 is a truck 904 which comprises a cab904A and a trailer 904B. The sensor array 912 will detect the cab 904Aand the trailer 904B as separate objects, however as the spacing betweenthe two components of the vehicle 904A, 904B is small and the directionand speed of the two components are identical, the system is able todetermine correctly that the vehicle 904 is a single truck.

In a second lane 910 of the road 902, a vehicle 906 is not travellingparallel to the lane and in the direction of traffic flow, but isinstead travelling at an oblique angle 950 in order to change lanes.While conventional systems would not be able to resolve such drivingpatterns, the data from the sensor array 912 is used to create abounding box at any angle from which parameters relating to the vehicle906 such as its length, width, and speed from which classification ofthe vehicle can be determined. The system is also able to determine thedirection in which the vehicle 906 is travelling as well as the angle950 relative to normal traffic flow at which it is travelling.

FIG. 10 shows the relative spacings 1010, 1016 between the sensors 114compared to vehicle sizes in accordance with an embodiment of theinvention. A sensor array 1012 comprises a number of individual sensors114 as has been described previously arranged in an interleaved ordiamond pattern. These sensors 114 are spaced apart with a longitudinalseparation distance 1010 normal to the direction of traffic flow ofe.g., one metre and a transverse separation distance 1016 parallel tothe direction of traffic flow of e.g., 0.25 meters. These figures are ofcourse purely exemplary. The entire sensor array 1012 has a lateralwidth 1030 extending across the road normal to the direction of trafficflow and a longitudinal length 1032 parallel to the direction of trafficflow.

For illustrative purposes a motorcycle 1004, a car 1006, and a truck1008 (comprising a cab 1008A and a trailer 1008B) are shown. In thisparticular example, the motorcycle 1004 is the narrowest vehicle that isto be detected. Accordingly, the lateral separation distance 1010 mustbe less than the width of the motorcycle 100018, but the width of theentire array 1030 must be greater than the width of the motorcycle 1018.In this instance, the width 1030 of the array 1012 is in fact greaterthan the width 1026 associated with the truck 1008, the widest vehicleto be detected. The car 1006 with width 1022 falls between these ranges.

The longitudinal separation 1016 between the individual sensors is alsoless than the minimum length to be detected, in this case themotorcycle's length 1020. However, similarly to the width requirements,the length of the array 1032 is greater than the length 1028B of thetruck trailer 1008B, the longest vehicle to be detected. The car 1006with length 1024 and the truck cab 1008A with length 1028A fall betweenthese ranges.

It can be seen therefore that the motorcycle 1004, car 1006, and truck1008 each fit within the sensor array 1012, but the spatial resolutionof the sensors 114 is sufficiently fine to be able to resolve each ofthe vehicle classifications. It will be appreciated that while the truck1008 fits within the sensor array 1012 in its entirety, the system canadequately detect and classify the truck 1008 when the array 1012 islonger than the individual cab 1008A and trailer 1008B but shorter thanthe combination of the two.

Thus it will be seen that a vehicle detection and classification systemhas been described. Although a particular embodiment has been describedin detail, many variations and modifications are possible within thescope of the invention.

The invention claimed is:
 1. A vehicle detection and classificationsystem comprising a plurality of ultra-wideband proximity sensorsdistributed in a fixed spatial array relative to a road, wherein eachsensor is spaced from a nearest adjacent sensor by a respective distanceless than a minimum horizontal dimension of a vehicle to be detected butthe array has a maximum dimension greater than said minimum horizontaldimension of a vehicle to be detected, each of said sensors beingconfigured to determine presence or absence of a vehicle and tocommunicate data regarding said presence determination to a dataprocessing system and wherein the ultra-wideband sensors can communicatewith each other using ultra-wideband signals, said data processingsystem being configured to use data from a plurality of said sensors todetect and classify a vehicle on said road based on at least onedimension of said vehicle.
 2. The vehicle detection and classificationsystem as claimed in claim 1 wherein the array of sensors is provided inone or more planes parallel to a surface of the road.
 3. The vehicledetection and classification system as claimed in claim 2 wherein atleast part of the array of sensors is disposed beneath or flush with asurface of the road.
 4. The vehicle detection and classification systemas claimed in claim 1 wherein at least some of the sensors areconfigured to communicate said data to said data processing system viaat least some other sensors.
 5. The vehicle detection and classificationsystem as claimed in claim 1 wherein said data processing system isarranged to determine one or more of: a length; a width; a speed; or anangle of travel of the vehicle.
 6. The vehicle detection andclassification system as claimed in claim 1 arranged to determine adistance to the vehicle.
 7. The vehicle detection and classificationsystem as claimed in claim 1 arranged to detect axle usage.
 8. Thevehicle detection and classification system as claimed in claim 1wherein the data processing system is arranged to determine a number ofaxles present on the vehicle.
 9. The vehicle detection andclassification system as claimed in claim 1 wherein at least part ofsaid data processing system is distributed across the array of sensors.10. The vehicle detection and classification system as claimed in claim1 wherein at least part of the array of sensors is disposed above asurface of the road such that vehicles pass beneath the sensor.
 11. Avehicle sensing system comprising a plurality of ultra-widebandproximity sensors distributed in a fixed spatial array relative to aroad, wherein each sensor is spaced from a nearest adjacent sensor by arespective distance less than a minimum horizontal dimension of avehicle to be detected but the maximum dimension of the array is greaterthan said minimum horizontal dimension of a vehicle to be detected, eachof said sensors being configured to determine presence or absence of avehicle and in use to communicate data regarding said presencedetermination to a data processing system, and wherein theultra-wideband sensors can communicate with each other usingultra-wideband signals, said data being such as to allow said dataprocessing system to use data from a plurality of said sensors to detectand classify a vehicle on said road based on at least one dimension ofsaid vehicle.
 12. The vehicle sensing system as claimed in claim 11wherein the array of sensors is provided in one or more planes parallelto a surface of the road.
 13. The vehicle sensing system as claimed inclaim 12 wherein at least part of the array of sensors is disposedbeneath or flush with a surface of the road.
 14. The vehicle sensingsystem as claimed in claim 11 wherein at least some of the sensors areconfigured to communicate said data to said data processing system viaat least some other sensors.
 15. The vehicle sensing system as claimedin claim 11 wherein said data processing system is arranged to determineone or more of: a length; a width; a speed; or an angle of travel of thevehicle.
 16. The vehicle sensing system as claimed in claim 11 arrangedto determine a distance to the vehicle.
 17. The vehicle sensing systemas claimed in claim 11 arranged to detect axle usage.
 18. The vehiclesensing system as claimed in claim 11 wherein the data processing systemis arranged to determine a number of axles present on the vehicle. 19.The vehicle sensing system as claimed in claim 11 wherein at least partof said data processing system is distributed across the array ofsensors.
 20. The vehicle sensing system as claimed in claim 11 whereinat least part of the array of sensors is disposed above a surface of theroad such that vehicles pass beneath the sensor.